148 METALLURGY OF IEON AND STEEL. eye, deep-blue glasses "being used as a protection from the intense glare. I have elsewhere* shown that the practiced eye can detect a difference of 13° C. in the temperature of Bessemer charges., and tins may also be taken as the skill to which many open-hearth melters attain. The intense heat of a regenerative furnace is made possible by the preheating of the gas and air in chambers which have been warmed by the products of combustion,, these chambers being alternately heated by currents traveling from the furnace to the valves, and cooled by currents going from the valves to the furnace. If the currents were not reversed, the chambers on the outgoing end would be heated uniformly throughout their length to about the temperature of the furnace, while, at the same time, the chambers on the incoming end would be cooled to the temperature of the incoming gases. By the reversal of the currents there is a continual conflict between these extremes, so that the ends next the melting chamber are at a bright yellow heat, and the ends next;the valves are about 200° F. (say 100° C.) above the temperature of the incoming gases. .. Air always enters cold, but "it is believed by some furnacemen that it is economical to have the gas as hot as possible. To some extent this is an error, for the checkers in the outer end of the gas chamber cannot be cooled below the temperature of the entering gas, and the products of combustion cannot be cooled below the temperature of these checkers, so that the heat carried in by hotter fuel is carried out by hotter waste gases, and no economy is obtained. With hot gas, however, it is not necessary to pass such a, large proportion of the products of combustion through the gas chambers, and an extra amount may be diverted to the air chambers, where the heat may be used to advantage. This gain may be important when the coal contains only a. small proportion of the denser hydrocarbons, for under these conditions the gas leaves the producer at a high temperature; but when the coal is very rich the gas is at a low temperature when it comes from the fire, and the gain from its immediate use may be inappreciable. It is true that all the tar is utilized when hot gas is used, but this represents only a small part of the total calorific development.. SEC. Vllli.—Calorific equation of an open-hearth furnace.—• * The Open-Hearth Process. Trans. A. I. M. 22., Vol. XXII, p. 392. See also certaiq remarks in Sec. Vli.